easiest way to figure that out is to google a wattage calculator. then add at least 30% to the figures you get, and that should be the minimum wattage for the PSU. and make sure you stick to a top brand. look for enermax, antec, corsair, and a few others. you can refer to this list if you like. that will help you pick out the PSU once you have your figures.

It's not a bad thing if the calculators think you need more power as there's no such thing as having too good a power supply. It will only give your PC the power it needs, so if you had a system that needs 300W and you bought a 1000W PSU that would be fine as it'll give the PC 300W. Although it would obviously cost more to buy and is overkill really